(1) Field of the Invention
The present invention relates to an analog to digital converting device having high resolvability wherein an analog signal is converted to a multi-bit digital signal by means of quantizing the analog signal by minute level steps.
(2) Description of the Prior Art
In order to perform analog to digital conversion with high resolvability, it is customary to commence the procedure of conversion with the lowest resolvability caused by quantizing the analog signal with use of the most coarse quantizing steps, and then to perform successive procedures of conversion with successively higher resolvability owing to successively finer quantizing steps, those procedures being followed by combining respective results thereof and by appropriate code conversion, so as to form a multi-bit digital signal having the required high resolvability and the desired digit configuration.
An analog to digital converting device provided for effecting the above-mentioned procedures is constructed as a multi-stage converter consisting of at least two analog to digital converting stages, wherein the upper order stages takes shares in the respective procedures relating to the successively lower resolvability, and the lower order stages take shares in those relating to the successively higher resolvability.
The lower the order of the converting stage is, the finer its quantizing step. Accordingly, it is required for the purpose of effecting the analog to digital conversion with sufficiently high accuracy in the lower converting stage, to stabilize the procedure of conversion effected in the upper stages, especially, in the just preceding stage by preventing deterioration in accuracy caused by DC-drift and gain-drift of circuit components contained in those preceding stages. As a result, a conventional multi-stage analog to digital converting device has a complicated configuration for respective converting stages for cancellation of or compensation for the above-mentioned DC-drift and gain-drift of circuit components.
FIG. 1 shows a portion, that is, the last and a just preceding converting stage of the conventional multistage analog to digital converting device having the high resolvability. In these converting stages, which are separated by a chain line A-A in FIG. 1, an input analog signal, which is applied to an input terminal 1 by a further preceding converting stage, is fed to an analog to digital converter 2 in the upper converting stage, wherein the input analog signal is converted into an upper bit digital signal corresponding to coarse quantizing steps. The upper bit digital signal derived from the converter 2 is applied to a digital to analog converter 3 in the lower stage, wherein the digital signal is reconverted into an analog signal which is equivalent to the upper bit digital signal. The reconverted analog signal is applied together with the input analog signal to a subtracter 4, which develops a differential analog signal from analog signals applied thereto. The differential analog signal derived from the subtracter 4 is applied through an amplifier 5 to an analog to digital converter 6, wherein the differential analog signal is converted into a lower bit digital signal corresponding to the fine quantizing steps. The upper and lower bit digital signals formed respectively as described above are fed together to output terminals 7, where an output digital signal consisting of those signals is derived.
In the configuration mentioned above, it is required that the DC level stabilization of the converters 2 and 3, the subtracter 4 and the amplifier 5 and respective gains of those circuit components be fixed onto respectively prescribed values with extreme stability. Otherwise, the above-mentioned differential analog signal applied from the amplifier 5 to the analog to digital converter varies from a proper level, so that the lower bit digital signal derived from the converter 6 becomes incorrect.
For the purpose of preventing the deterioration of accuracy and stability of conversion, especially the lower bit conversion, caused by the instability of circuit components, and of stabilizing the whole converting device, various kinds of measures have been contrived. However, those measures result in complicated configurations having insufficient accuracy and reliability.
The problems described above are serious and are common to various kinds of multi-stage analog to digital converting devices.